Given a flow network G = (V, E), let f1 and f2 be functions from V × V to R. The flow sum f1 + f2 is the function from V × V to R defined by (26.4) (fi + f2) (u, v) = f1 (u, v) + f2(u, v) for all u, v ¬ ⁪ V. If f1 and f2 are flows in G, which of the three flow properties must the flow sum f1 + f2 satisfy, and which might it violate?